This invention relates to optical switching networks and, more particularly, to optical switching network architectures that reduce crosstalk noise through judicious routing of signals.
Wideband optical signals can be switched with electronic control using electrooptic waveguide couplers using TiLiNbO.sub.3 waveguides on a planar LiNbO.sub.3 crystal. The basic switching element is a coupler with two active inputs and two active outputs. Depending on the amount of voltage at the junction of the two waveguides which carry the two input signals, the two inputs can be coupled to either of the two outputs. Several architectures have been proposed to construct an N.times.N switch with the 2.times.2 directional coupler as the basic component. These architectures are essentially analogs of similar architectures for electronic switching and interconnection networks. However, due to the difference in characteristics of the electronic and optical switching elements, performance of the optical architectures is significantly different.
Specifically, regeneration of signals in optical systems is difficult. This difficulty leads to the desire to reduce losses and eliminate noise sources so that the need for regeneration can be diminished. In light of this desire, some architectures that are useful when realized with electronics are less favored for optical realizations.
The attenuation of light passing through a waveguide optical coupler switch has several components: (a) fiber-to-switch and switch-to-fiber coupling loss, (b) propagation loss in the medium, (c) loss at waveguide bends, and (d) loss at the couplers on the substrate. Often, the last factor predominates and, therefore, a substantial part of the attenuation in a switch fabric is directly proportional to the number of couplers that the optical path passes through.
Optical crosstalk results when two signals interact with each other. There are two primary ways in which signals flowing in optical paths can interact in a planar substrate. First, the channels (waveguides) carrying the signals could cross each other in order to imbue a particular topology and the interaction occurs in the neighborhood of the crossover. Secondly, two paths sharing a switching element experience some undesired coupling from one path to the other. We call the former path crossover crosstalk, and the latter switch crossover crosstalk. The easiest way to reduce path crossover crosstalk is to reduce the size of the neighborhood within which the interaction occurs. This can be accomplished by keeping the intersection angles above a certain minimum amount. A more difficult approach is to make the interaction neighborhood precisely long enough to couple the signal entirely from one path to the other and back. With path crossover crosstalk reduced to a negligible level, switch crossovers remain as the major source of crosstalk in optical switching networks constructed out of electrooptic waveguide couplers.
The effort to reduce crosstalk is hence directed at the switch design, at the number of switches that a particular architecture requires in the optical path, and at the signal flow patterns within the network.
In the IEEE Transactions on Communications, Vol. COM-35, No. 12, December 1987, we published an article titled "Dilated Networks for Photonic switching". The article presents a number of networks where the third approach to reducing crosstalk is disclosed. We call this the dilated networks approach. Specifically, we disclosed a design for the Benes network that, while maintaining the rearrangeable non-blocking characteristic of the network, insures that no 2.times.2 switch in the network has optical signals applied concurrently to both its inputs. We also mentioned that a corresponding design for the Omega network is possible.
What we have not described, however, is a method for modifying the structure of any given network to create a corresponding network that is dilated.